1. Field of the Invention
This invention relates to the field of dynamic magnetic information storage and/or retrieval, and more specifically, to a record transport with one or more rotating heads which record and/or reproduce machine-convertible information while moving, i.e., rotating, in transducing relationship with a magnetic web or tape; this information being oriented as magnetic domains to form information tracks which extend generally traverse to the longitudinal tape length.
2. Prior Art
Rotating head magnetic tape units are widely known. In one form of prior art device, a tape guiding structure in the form of a generally cylindrical mandrel or drum includes a rotating head wheel which carries one or more read/write heads. The magnetic tape engages the mandrel at one point, makes a helical wrap about at least a portion of the mandrel and exits the mandrel at a point which is both axially and circumferentially spaced from the entrance point. The angle of helical tape wrap can vary in accordance with design choice, but is usually between 180.degree. and 360.degree.. The head wheel rotates so as to sweep its magnetic head or heads traversely across the tape. The angle at which the head enters and exits the tape may vary in accordance with design choice, slightly less than 90.degree. to a small angle such as 15.degree..
Another form of prior art device is one wherein the head wheel is associated with a tape guiding structure which bends the tape traversely into an arcuate shape that conforms to the circumferential shape of the head wheel. In this device the tape travels in a generally straight line past the head wheel, and is traversely bent by the associated guide as it enters the head wheel area.
The present invention finds utility with either aforementioned type of device, and has been found particularly useful with the helical wrap device.
Generally, in either of the above mentioned devices, the rotating head wheel has one or more magnetic transducers which read or write data on the magnetic media. This head wheel is attached to a rotating structure and the combination is called the rotor. The rotor communicates with the fixed section or stator of the rotating head magnetic tape unit via conductive or transformer interface.
The aforementioned prior art rotating head tape units are plagued with several problems. One of the major problem encountered with these devices is that of the frequency band limited nature of the rotating interface in conjunction with the magnetic transducers. This frequency band limitation is due to the typical inductive nature of the magnetic transducer, stray cabling capacitance and the reactive nature of the rotating coupler. If the rotating coupler is a transformer, physical packaging constraints limit the achievable high and low frequency response. If the rotating coupler is slip-rings or bushes, noise is an additional factor limiting high frequency response. Capacitive couplers suffer similar problems to transformers.
This frequency band limitation has four distinct problems: (1) The limited low frequency response constrains the code selection to those codes with no low or zero frequency components in their power spectrum. These are typical codes of the frequency modulation (F.M.) family which require higher channel bandwidths to pass a given information data rate. (2) The high frequency limitations constrains the allowable information rate which for rotary head devices is low compared to modern computing systems; typically 5-15 megabits in prior art systems. (3) The pass band limitation also results in phase distortion of which the low frequency is non-correctable. This distortion, in a digital recording channel, manifests itself as bit shift causing reproduce errors. This limits the achievable recording linear density and/or readback reliability. (4) The readback or reproduce amplifier is remote from the reproduce transducer and is connected to the transducer via the reactive rotary coupler. To achieve high bandwidth, normally the transducer impedance must be reduced to compensate for coupler leakage inductance and cable capacitance. This is at the expense of readback signal amplitude. Also the reactive nature of the coupling network presents considerably more noise (both thermal and environment) to the amplifier. Therefore a significant decrease in signal-to-noise ratio occurs over that which would be obtainable if the read amplifier were directly matched to the read transducer. This again limits achievable linear recording densities and/or readback reliability at any given data rate.
The frequency limitations of the rotary couplers also limit the use of A.C. (alternating current) Bias Write recording. High density recording, approximately 20,000 flux changes per inch or greater, appears to require A.C. Bias Write if satisfactory reliability is to be obtained. This A.C. Bias Write requires the channel to pass a frequency several times higher than the highest data rate to the write head. When the maximum frequency obtainable is limited, if A.C. Bias Write is desired, then data rate must be decreased.